Well, that might be a little exuberant; maybe an IRC excerpt
isn't the best way of summarizing an achievement. So what
have we done? Well, we now have a Common Lisp compiler,
written in Common Lisp that can be built from a mostly
unrelated Lisp compiler.

To people in the C world, this may not seem unusual. After
all, gcc is built initially by vendor C compilers, then by
itself. The difference between compiling C and compiling
Lisp (well, OK, a difference) is that the act of
compilation changes the state of the compiler.

This therefore raises portability issues as soon as you try
to model the act of compilation itself. For instance, CMUCL, in its build
process, scribbles over its own internal data structures,
which is fine, as long as you're not trying to change the
representation of those data structures; if you are, you
need to find some way of bootstrapping the changes.

As of this week, though, we can state with a small degree of
confidence that one can make arbitrary (consistent) changes
to the source of SBCL and not have to
deal with the bootstrapping question. What does this buy us?
Nothing, really, except a small degree of confidence that
one can change various representations without having nasty
surprises jump out at us. This is not an end-user-visible
improvement, really. It does give the maintainers a warm,
fuzzy feeling, though.